Side Channel Restoration, Prioritization and Design
Feb 20, 2021
study objectives
Many historic Big Horn River side channels currently only flow during high water because they have lost most of
their connectivity with the main channel. This loss of connection is mostly due to sediment deposition and vegetation encroachment at their entrances, such that the heads of the channels are too high and too densely vegetated to frequently carry water. As these side channel habitats are recognized as important contributors to the health of the Bighorn river fishery, a recent focus of the Bighorn River Alliance has been to explore the feasibility of their reactivation.
The intent of this effort was to assess side channel restoration opportunities from Yellowtail Dam to the Yellowstone River and develop permit-ready designs for top-tier projects.
findings
Initially, dozens of channels were considered for detailed assessment, but many were immediately discarded due to poor feasibility caused by excessive perching, infrastructure complications, land use complications etc. Ultimately, a total of 13 channels were evaluated between Afterbay Dam and St Xavier, 7 between St Xavier and Hardin, and 10 below Hardin. Of these 30 channels, 13 were considered “top tier”. Ultimately, Juniper and Rattlesnake side channels were selected for a first phase of restoration implementation. During the summer of 2020 they were surveyed in detail and permit-ready designs are complete.
The estimated excavation volume for Rattlesnake Channel is about 130 cubic yards, whereas Juniper Channel will require about 419 yards of excavation to create an appropriately sloped, 20-foot wide channel through its course. Excavated materials will be used to create gravel bars along the length of each side channel, which are intended to keep the low-flow width of the side channels relatively narrow. The entrance to both channels is designed with a departure angle between 40 and 50 degrees to encourage gravels to flush rather than deposit.
implications
The project is currently in the permitting phase, with construction slated for Spring 2021. The projects will reactivate 1,800 feet of channel length on Juniper Channel and about 1,100 feet on Rattlesnake Channel. Whereas Juniper channel currently flows at about 2,900 cfs, with the excavation it should activate at less than 2,000 cfs. Current activation flows on Rattlesnake channel are similarly about 2,900 cfs; excavation of its entrance should increase connectivity so that it will flow at around 2,200cfs.
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